00777191 Axial Flux Pmg Toroidal

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832 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 35, NO. 4, JULY/AUGUST 1999

Fig. 1. One pole of the stator and rotor core.

Section II introduces the generator components. In Section III,

we present our analysis of the PM generator. In Section IV,

we describe testing and, lastly, in Section V, the conclusions

are summarized.

II. COMPONENTS OF THE GENERATOR

In this paper, we discuss only one unit module of the

generator. The generator consists of an 18-pole PM. The stator

and the rotor cores are made of precut transformer lamination

silicon steel (gauge 26, M19). The stator and rotor cores can

be made on a per-pole basis, reducing the cost of complete

dies required to stamp a conventional lamination configuration.

The geometry of the stator and the rotor core could have

been optimized, however, this project focuses on the proof

of concept.

A. RotorThe cross section of the stator and rotor pole is illustrated in

Fig. 1. Each pole is constructed from two identical core stacks

and the PM is sandwiched in between. The rotor is constructed

to allow an expansion in the axial direction, for example, to

increase the magnet surface. The flux directions at the top

(outer radius) and the bottom (inner radius) of the rotor pole

are the opposite. Around the perimeter of the rotor, the flux

direction of one pole is opposite of the flux direction in the

next pole, as shown by the white arrows in Fig. 2. The ratio of

the magnetic surface area to the pole surface area determines

the focusing factor. The chosen geometry enables the designer

Fig. 2. PM generator with toroidal winding.

to increase the length of the rotor core without affecting the

stator geometry and vice versa.

The rotor poles are attached to a nonmagnetic disk thatholds the rotor cores. The shaft is attached to the disk to rotate

the rotor core. A nonmagnetic stainless steel belt is strapped

around the rotor core to keep the rotor poles in place. Since

the rotor speed is low, centrifugal force created when the rotor

rotates is not very high. There are nine pole pairs on the rotor.

Between two rotor poles, there is a small gap to minimize

interpolar magnetic leakage.

B. Stator

The stator consists of two stator sides. There are nine poles

attached to each stator side. The poles on each side are attached

to a plate (not shown in Fig. 2) which holds the stator polesin place. In the prototype, one side of the stator core can be

rotated (within a limited angle range) with respect to the other

stator side. Thus, the position of the stator cores in one side

can be shifted with respect to the other sides. The shift can

be adjusted to control the phase shift between the first stator

side and the second stator side.

C. Stator Winding

The stator winding is wound like a torus or a washer. With

a toroidal form, the stator winding can be easily assembled

and automated for production. The stator winding between the

stator poles is exposed to open air, which improves cooling.

In general, the load to the wind turbine is regulated as acube function of the rotor revolutions per minute to operate the

wind turbine at the optimum efficiency. The tip-speed ratio (the

ratio of the blade-tip speed to the wind speed) should be kept

constant at the optimum power coefficient of the wind turbine.

With this load characteristic, as the wind speed increases, the

load is regulated to follow the wind speed. Thus, the load is

increased as the wind speed increases.

One advantage of wind power systems is the location of the

generator. It is mounted on a tower above the ground. The

cooling mechanism is better up on the nacelle than inside a

ground-level building because the generator is always exposed


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MULJADI et al.: AXIAL-FLUX MODULAR PM GENERATOR WITH A TOROIDAL WINDING 833

Fig. 3. Expansion for multimodule generation.

to air flow that is proportional to the generator load. Duringlow wind speeds, the heat transfer from the winding is lower;

however, the heat generated in the winding is lower, too. The

opposite is true at high wind; more heat is generated in the

winding, but more air flow is available to transfer the heat

away.

We built one module unit for a single-phase generator. The

stator windings at the two sides are connected in parallel to

generate a single-phase output. The rotor shaft is attached to

the stator sides through the bearings, which are attached to the

stator plate. The rotor core has a width of 6.35 cm (2.5 in)

and a diameter of 29.2 cm (11.5 in). The overall width of the

generator is 16.5 cm (6.5 in), excluding the two stator plates.

The weight of one stator pole unit is 217 g (or 3.9 kg for 18poles). The weight of the stator winding for one side is 2061

g or 4122 g for both sides. The weight of one rotor pole unit

is 612 g, including the PM (or 11 kg for 18 poles).

D. Expansion for Multimodule Generation System

The power from the stator can be actively controlled using

power switches [insulated gate bipolar transistors (IGBTs)]

or passively controlled using a diode rectifier. Fig. 3 shows a

possible configuration of the power converter to process the

power generated by the generator. The generator may consist

of one or more modules. In this configuration, only three

unit modules are shown. Each unit module of the generatoris paired with one leg module of power switches on the power

converter side. Thus, the power converter and the generator

can be expanded in a similar fashion. The power generated is

converted back to the utility via a three-phase inverter, which

can be controlled to produce good power quality.

III. DESIGN ANALYSIS

The analysis of the generator is based on the wind-turbine

requirements. The steady-state analysis was performed as

the first step to get the first cut of design criteria. The

finite-element analysis was performed to refine the magnetic

analysis. Finally, a dynamic analysis was performed in the

laboratory to validate generator performance under dynamic

conditions.

A. Steady-State Analysis

The prime mover for this generator is a wind turbine. One

characteristic of wind turbines is that the rotational speed is

lower than most prime movers. To avoid using a gearbox,the generator is direct driven. Multiple poles must be used to

allow slow-speed operation.

Most horizontal axis wind turbines operate at about the

same tip-speed ratio range. Larger turbines use a larger blade

radius. Thus, for the same wind speed range, the larger turbines

require a lower rotor speed than smaller turbines. For example,

a 300-kW turbine operates at about 40 r/min; the 10-kW

turbine operates at about 400800 r/min, depending on the

choice of the blade radius range of tip-speed ratio. In the

preliminary design presented in this paper, the 667-r/min speed

is not an optimum design.

From steady-state analysis, the following criteria are chosen:

number of poles 18maximum operating

frequency 100 Hz (at 667 r/min)number of phases

per unit module 1 (two windings in parallel).

The electric loading:

stator current 11.0 A rms (at per-phase volt-

age 58-V rms)

wire chosen AWG 12

current density

in the slot J 3.4 10 A/m

predicted copper losses

at rated current 42 W.

B. Finite-Element Analysis

To analyze the magnetic circuit, the finite-element method

was used to compute the flux density in the generator com-

ponents. The main purpose of this analysis is to get the

overall picture of the saturation levels in different parts of the

generator, the iron losses in the components of the generators,

and the worst case of demagnetization on the permanent

magnet. In the finite-element analysis presented here, the

generator uses a ferrite PM.

1) No-Load Condition: In the no-load condition, the mag-

netic path is analyzed to see the magnetic flux density in

different parts of the magnetic paths. With the stator corein each side shifted by 180 , the maximum flux in the core

happens when the stator core and the rotor core are aligned.

Fig. 4 shows the flux lines at the no-load condition. Only one

side of the stator core is shown. Some flux leakage is shown,

such as at both ends of the rotor poles. The rotor core has low

flux density with the highest flux density at the parts closest

to the air gap. As shown in Fig. 4, the maximum flux density

occurs at the corner of the U-shaped stator core. Fig. 5 shows

the magnitude of the flux density along the horizontal line in

the middle of the air gap. The maximum flux density at no load

is 1.55 T. The flux density at the air gap is 0.9 T and the flux


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Fig. 4. Flux lines at no-load condition.

Fig. 5. Flux density at no-load condition.

density at the permanent magnet is 0.24 T. The stator core and

the rotor core have a flux density below the saturation point.

2) Inductive Load at Rated Current: In this condition, the

magnetic path is analyzed to see flux reduction at the air gap

at the least favorable power factor. The generator is loaded to

have rated current.

3) Short-Circuit Condition: In this condition, the magnetic

path is analyzed to see the demagnetization effects on the PM.

In order to analyze the worst case scenario, the stator core

and the rotor core are perfectly aligned and the short-circuit

current is applied to the stator core. In this case, the short-

TABLE IFLUX-DENSITY COMPARISON AT DIFFERENT

MAGNETIC PATHS FOR DIFFERENT CONDITIONS

Fig. 6. Open-circuit voltage.

circuit current is about ten times the rated current. The result

is tabulated in Table I.

IV. EXPERIMENTAL RESULTS

A. Experimental Setup

The experiment was conducted to observe the performance

of the generator. The generator is driven by a motor via a

belt. The motor is a four-pole motor, with rated speed of 1800

r/min. The motor is fed by a pulsewidth modulation (PWM)variable-frequency drive. The generator speed is driven to 667

r/min. The output frequency at this speed is 100 Hz. The

experiment is conducted only on a single unit generator. In

the finite-element analysis, the PM used is ferrite; however, in

this experiment, the PM chosen is a rare earth PM (NdFeB).

B. Voltage and Current Waveforms

The open-circuit voltage is measured at the terminal output

of winding 2 (open circuit). The stator cores are shifted toward

each other by 180 electrical degrees. The voltage waveform is

captured from the scope, digitized, and plotted in Figs. 6 and

7. In Fig. 7, the generator is loaded with resistive load up to

rated load at 100 Hz. The voltage across the terminal output

of the generator is a unity power factor load. Thus, the current

waveform is reflected by this terminal voltage waveform.

C. Parameter Determination Test

A simple modified test is used to get the parameters of

the PM. The experiment is shown in Fig. 8. One side of the

generator (winding 1) is connected to a rated load at unity

power factor. The generator is driven to generate a rated

frequency. The other side of the winding (winding 2) is an

open circuit. The voltage output of winding 1 is called terminal

voltage and the open-circuit voltage of winding 2 is called

open-circuit voltage . The angle difference between and


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MULJADI et al.: AXIAL-FLUX MODULAR PM GENERATOR WITH A TOROIDAL WINDING 835

Fig. 7. Terminal voltage across resistive load.

Fig. 8. Experimental setup.

TABLE IIRESULTS FROM TEST DATA

is called , which is the torque angle of the generator at

this load. The power, current, and voltage output of winding

1 are recorded.

The parameters can be computed from the test data, and

the results are listed in Table II. The efficiency is about

75%, which is quite reasonable for a small and nonoptimized

design. The efficiency can be increased by improving the

magnetic design (minimizing the leakage and core losses) and

mechanical design (lower friction and windage losses).

V. CONCLUSION

We have investigated the proposed generator for application

in wind-power generation. In the first stage of implementation,

a proof of concept of the generator was investigated. The

magnetic and electric loading were shown to be within the

limits of common practice of machine design. The gener-

ator has the following advantages for wind-turbine genera-

tion.

The modular concept is suitable for commercially produc-

ing limited quantities of machines with different sizes and

output requirements. The components are manufactured

on a per-pole basis. The tooling required is minimized.

The design can be readily changed, such as the number

of poles in one unit or the number of unit modules in a

generator system. Although, in this paper, the mechanical

design is not the main objective of the work, the mechan-

ical construction of this generator was a challenge. The

structure must be built to withstand any unbalanced axial

forces that can cause the rotor to wobble. A significant

mechanical design improvement can be made to the

present design.

The winding is torroidal in shape. Preformed winding can

be utilized and the winding can be inserted in the stator

core. For a low-volume production, the winding can be

done manually. For high-volume production, automaticaxial machine winding would be more suitable.

The axial-flux design makes it easier to increase the flux

density in the air gap.

To scale up the output power of the generator, more units

can be stacked in the axial directions. The power con-

verter required to process the power is readily compatible

with the generator. Each unit module of the generator is

matched with each leg of the power switches.

ACKNOWLEDGMENT

The authors wish to thank J. Bianchi for his assistance

during the test setup and J. Adams for his help during thefabrication of this generator. They also wish to acknowledge

the management at the National Renewable Energy Laboratory

(NREL) and the U.S. Department of Energy (DOE) for their

encouragement and approving the time and tools needed for

this project.

REFERENCES

[1] B. J. Chalmers and E. Spooner, An axial-flux permanent-magnetgenerator for a gearless wind energy system, in Proc. PEDES96, NewDelhi, India, Jan. 1996, pp. 610612.

[2] E. Spooner and A. Williamson, Modular, permanent-magnet wind-turbine generators, in Conf. Rec. IEEE-IAS Annu. Meeting, San Diego,

CA, Oct. 610, 1996, vol. 1, pp. 497502.[3] E. F. Fuchs, A. A. Fardoun, P. Carlin, and R. W. Erikson, Permanent

magnet machines with large speed variations, in Proc. Windpower92,Seattle, WA, Oct. 1992, pp. 291299.

[4] F. Carrichi, F. Crescimbini, and F. Mezzetti, Multistage axial-flux PMmachine for wheel direct drive, IEEE Trans. Ind. Applicat., vol. 32, pp.882887, July/Aug. 1996.

[5] S. Huang, J. Luo, and T. A. Lipo, Analysis and evaluation of thetransverse flux circumferential current machine, in Conf. Rec. IEEE-IAS

Annu. Meeting, New Orleans, LA, Oct. 59, 1997, vol. 1, pp. 378384.[6] R. Marsh, R. Chrenko, C. R. Gamble, D. Heberte, W. Holley, B. Javier,

D. Koconis, R. Millis, T. Lipo, Y. Li, and F. Leonardi, Direct-drivewind turbine feasibility study, survey of turbine technologies, presentedat the AIAA Conf., Reno, NV, Jan. 5, 1997.

[7] Direct-drive wind turbine feasability study, Electric Power ResearchInstitute, Palo Alto, CA, Rep. EPRI TR-106794, Apr. 1996.


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Eduard Muljadi (S83M84SM94) received theM.S. and Ph.D. degrees in electrical engineeringfrom the University of Wisconsin, Madison.

From 1988 to 1992, he was with the Departmentof Electrical and Computer Engineering, CaliforniaState University, Fresno, as an Assistant and Asso-ciate Professor. In June 1992, he joined the NationalRenewable Energy Laboratory, Golden, CO. Hiscurrent research interests are in the fields of electricmachines and power electronics, with emphasis on

renewable energy applications. He is the holder oftwo patents in power conversion for renewable energy.

Dr. Muljadi is currently a member of the Industrial Drives, ElectricMachines, and Industrial Power Converter Committees of the IEEE IndustryApplications Society. He is also a member of Eta Kappa Nu and Sigma Xi.

C. P. Butterfield received the B.S. and M.S. degreesin mechanical engineering from the University ofMassachusetts, Amherst.

He is the Manager of the Certification Program atthe National Wind Technology Center, National Re-newable Energy Laboratory (NREL), Golden, CO.He leads many technical review teams in evaluationof wind turbine designs. He managed the entireengineering development of two innovative windturbine designs that were considered state of theart at the time they were developed and deployed.

He designed and developed a comprehensive aerodynamics test and researchprogram that became the model for several international aerodynamic researchefforts. Before his current position, he was a Research Engineer for windturbine aerodynamic research, a Manager of the Wind Energy AppliedResearch Program, and a Chief Engineer in charge of technical review teamsand technical direction for the Turbine Development Program at NREL.Before joining NREL, he was the owner and Vice President of Engineeringfor ESI, a company that designed, manufactured, and operated utility-scalewind turbines. He was involved with test engineering and technical reviewsfor the small wind turbine research program at Rockwell International. Hehas authored more than 60 publications concerning aerodynamics, structuraldynamics, electrical control, and testing of wind turbines.

Mr. Butterfield was the recipient of the Technical Achievement Award fromthe American Wind Energy Association in 1983 and the NREL DirectorsAward in 1993.

Yih-huie Wan received the B.S. degree from Na-tional Cheng Kung University, Taiwan, R.O.C., andthe M.S. degree from Southern Illinois University,Carbondale, IL, in 1973 and 1979, respectively, bothin electrical engineering.

He is a Senior Engineer with the National Re-newable Energy Laboratory (NREL), Golden, CO.His main expertise is in electric power systemengineering, planning, and operation. Before joiningNREL, he worked as an Electrical Engineer for an

electric power company in Oklahoma for ten yearsin substation and distribution feeder design, transmission system planning, andbulk power transaction analysis. His work at NREL includes renewable energytechnology assessment and integration of renewable energy technologies withthe electric utility grid. He has also done work in renewable energy policyanalysis, renewable energy resource assessment, and distributed generation.

Mr. Wan is a Registered Professional Engineer in the State of Oklahoma.

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00777191 Axial Flux Pmg Toroidal